The present invention relates to a sound reproduction device or loudspeaker and especially one employing an electro-acoustic transducer. Another aspect of the invention relates to a microphone.
Loudspeaker driver units in common use today consist of a sound generation device, often a coil or wire in the field of a magnet, that operates a rigid acoustical diaphragm such as a cone. Loudspeakers in common use consist of one or more loudspeaker driver units mounted in a rigid enclosure so that one face of the acoustical diaphragm operates on the inside of the enclosure and other face produces the sound destined for the listener. The enclosure is often substantially air-impermeable although the enclosure may be vented to change the frequency spectrum characteristics of the sound produced. To date, most effort in the field of loudspeakers has been directed at the design of rigid enclosures and rigid acoustical diaphragms.
Whenever a diaphragm moves there is inevitably a region of compressed air at one face of the diaphragm and a region of rarefied air at the other face; indeed it is this property that is exploited in the common loudspeaker to produce sound. However, it is necessary to prevent the region of compressed air meeting the region of rarefied air, because if the two regions of air meet then they tend to cancel each other out and the overall sound level is reduced. In the common loudspeaker the enclosure and the acoustical diaphragm are usually sealed, but sometimes vented as discussed above, in order to prevent the compressed air at one face of the diaphragm reaching the rarefied air at the outer face of the diaphragm. In the common loudspeaker, it is usual for the enclosure and the acoustic diaphragm to be rigid to prevent the region of compressed air expanding into the area of rarefied air by deforming the enclosure or diaphragm.
A rigid enclosure introduces problems. Rigid enclosures must be made of materials such as plastic, metal, wood or wood composites, which are heavy and relatively expensive. The enclosure may be made of many individual pieces of material, tending to increase the cost of manufacture. It is sometimes necessary to reinforce the enclosures to prevent unwanted resonances, this makes the enclosure more difficult to manufacture. When a volume of air is trapped within an enclosure it tends to act as a sort of xe2x80x98acoustical springxe2x80x99 that resists the movement of the acoustical diaphragm, thereby decreasing the sensitivity of the loudspeaker and limiting its frequency response. A sound absorbing material is often employed inside the enclosure to help to reduce unwanted resonances or reflections, but this can reduce the sensitivity of the loudspeaker. Sometimes the loudspeaker may be weighted in order to further reduce unwanted resonances, but this tends to increase the costs of manufacture and distribution and it reduces the portability of the loudspeaker by the end user.
A rigid acoustical diaphragm also introduces problems. The mass of the diaphragm affects the frequency response and sensitivity of the loudspeaker, low mass diaphragms tend to make loudspeakers with a wider frequency range. Low cost diaphragm materials such as paper and canvas must be treated to increase their rigidity and to prevent absorption of water from the atmosphere, this increases the mass of the diaphragm and therefore reduces the frequency range. Rigid, low mass materials such as fiberglass or carbon fibre tend to cost more than paper or canvas and can be harder to manufacture.
When using a rigid acoustical diaphragm and a rigid enclosure, it is necessary for there to be a flexible coupling between the diaphragm and the enclosure. This coupling is usually made of high performance material that is substantially air-impermeable and must be able to withstand bending through a small radius. The flexible coupling must be able to bend many thousands of times per hour of use and survive the lifetime of the loudspeaker.
Most rigidly encased loudspeakers do not perform well in wet environments, for example if they have vented enclosures then water ingress is a problem. The low cost materials used in loudspeaker cones tend to absorb water, which damages them. Some loudspeaker driver units are provided with cones made of waterproof plastic films, but these tend to have poor sound reproduction qualities in comparison to other materials, and a sealed enclosure is also necessary.
Taking all these effects into account, the common loudspeaker appears to be a less than ideal solution. It would be desirable for there to be a loudspeaker that has an acceptable frequency response and sensitivity but avoids the restrictions of a rigid acoustical diaphragm and a rigid enclosure. That is an aim of the present invention.
Some attempts at flexible loudspeaker enclosures have been made in the past, most notably European Patent EP 0129320A1. This patent describes a flexible enclosure for a loudspeaker, but it relies on two principles, firstly it utilises the rigid cone of a standard loudspeaker driver unit and secondly it relies on the principle that one face of the cone acts on listener and the other face of the cone acts on the inside of the enclosure. The invention described in the above-mentioned patent is intended for use where it is impractical to provide a purpose-built enclosure for a loudspeaker, such as inside voice communication terminals.
The prior art in the form of rigidly enclosed loudspeakers are proven to be effective in domestic and industrial environments alike but they are expensive to manufacture and heavy, this is the opposite of what this invention sets out to achieve.
U.S. Pat. No. 5,108,338 proposes a balloon which has a music producing device, including a small speaker, affixed to the exterior wall of the inflatable balloon by way of an adhesive patch. The patch covers the speaker and mutes the acoustic output. The balloon enclosure itself does not function as an acoustic diaphragm.
Accordingly, one aspect of the present invention provides a sound reproduction device comprising an enclosure and an electrical transducer connected to the enclosure to impart movement thereto to generate sound and wherein the enclosure is made from a substantially air (gas) impermeable flexible material and wherein, for use, the enclosure is adapted to be stressed by internal pressure.
Since all sound reproducing devices (eg. loudspeakers) can be used in reverse as microphones, another aspect of the invention provides a microphone comprising an enclosure and an electrical transducer connected to the enclosure to generate an electrical signal in response to movement imparted thereto from the enclosure and wherein the enclosure is made from a substantially air (gas) impermeable flexible material and is adapted to be stressed by internal pressure.
More generally, the invention provides sound reproduction device or a microphone, comprising an enclosure and an electrical transducer connected to the enclosure to transfer movement therebetween, and wherein the enclosure is made from a substantially air (gas) impermeable flexible material and wherein the enclosure is adapted to be stressed by internal pressure.
Part of the enclosure acts as an acoustic diaphragm. More particularly there are usually two connections of the transducer with the enclosure and at least one of those connections coincides with the centre of a concavity formed in the wall of the enclosure. The concavity acts as the acoustic diaphragm. The formation of the concavity and the overall shape of the enclosure may be influenced by the configuration of the enclosure material. Struts, ties and gussets and/or localised reductions is flexibility of the enclosure material may be utilised to achieve the desired enclosure configuration. The enclosure is in the form of a flexible skin which is relatively thin and conveniently in the form of a film, say of plastics such as polyethylene or mylar. Preferably, a single material is used for the entire enclosure. Stressing is conveniently achieved by filing, eg. inflating, the enclosure with a fluid, for example a gas such as air, helium or carbon dioxide or with a liquid such as oil or water. Alternatively, a semi-solid may be employed. The semi-solid may be an open cell foam, or a jelly-like substance. Access to the interior for filing/inflating is by way of a suitable closable passage which may incorporate a one-way valve.
By using a material which is inflatable, the enclosure can be collapsed and for example folded into a small space when not in use. By using a material which is water impermeable, the loudspeaker can be used outside.
Conveniently the electrical transducer connects with the enclosure at least two locations, and preferably at two opposed locations whereby movements are transferred at both locations. More particularly the filling/inflation of the enclosure is such as to form two or more concavities so that each concavity behaves as an acoustic diaphragm. Each concavity is centred on its connection with the transducer. In the case of a loudspeaker the movement is imparted to the enclosure by the electrical transducers to generate sound, whereas in the case of a microphone the movement of the enclosure is imparted to the electrical transducer to generate an electrical signal representative of the sound.
The electro-acoustic transducer may be of moving coil, moving magnet, piezo-electric, electro-static or any other construction. A particularly simple construction results where a piezo-electric transducer is employed. A piezo-electric transducer has one or more rigid plates separated by a piezo-electric material that exhibits the property of changing thickness when an electric field is applied to it. Accordingly, fixing opposite ends of the piezo-electric transducer to opposite parts of the enclosure provides a means of imparting the sound generating movement to the enclosure. The enclosure may be formed from a piece or pieces of, for example, polyethylene film or more preferably still metalised mylar, which are joined or formed into a closed and inflatable envelope. Where transducers of other types are employed, means is provided for transmitting the motion of the transducers to the flexible enclosure at corresponding locations. Conveniently such other transducers are removed from the enclosure and the motion transmitting mechanism routed into the enclosure with appropriate sealing so that pressurisation of the enclosure is not jeopardised. The inflatable acoustic enclosure of the present invention may be incorporated into other article, especially other inflatable articles.
In the following description, the invention is described with reference to its application as a loudspeaker. For its application as a microphone the reference to xe2x80x9cmovement being imparted to the enclosure by the electrical transducerxe2x80x9d should be read as xe2x80x9cmovement of the enclosure being imparted to the electrical transducerxe2x80x9d. Thus, the connection between the electrical transducer and the enclosure could be regarded more generally a movement transfer mechanism.